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"Visual agnosia Case studies."
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A case study in visual agnosia revisited : to see but not to see two
\"Visual agnosia is a rare but fascinating disorder of visual object recognition that can occur after a brain lesion. This book documents the case of John, who worked intensively with the authors for 26 years after acquiring visual agnosia following a stroke. It revisits John's case over twenty years after it was originally described in the book To See But Not To See, in 1987. As in the previous book, the condition is illuminated by John and his wife, Iris, in their own words. A Case Study in Visual Agnosia Revisited discusses John's case in the context of research into the cognitive neuroscience of vision over the past twenty years. It shows how John's problems in recognition can provide important insights into the way that object recognition happens in the brain, with the results obtained in studies of John's perception being compared to emerging work from brain imaging in normal observers. This book presents a much fuller analysis of the variety of perceptual problems that John experienced, detailing not only his impaired object recognition but also his face processing, his processing of different visual features (colour, motion, depth), his ability to act on and negotiate his environment, and his reading and writing. A Case Study in Visual Agnosia Revisited will be a key reference for those concerned with understanding how vision is implemented in the brain. It will be suitable for both undergraduate students taking courses in cognitive psychology and neuropsychology, and also researchers in the cognitive neuroscience of vision. The presentation of John's case, and the human aspects of the disorder, will also be of great interest to a general audience of lay people interested in perception\"-- Provided by publisher.
Book
Saliency modulates global perception in simultanagnosia
Patients with parieto-occipital brain damage may show simultanagnosia, a selective impairment in the simultaneous perception and integration of multiple objects (global perception) with normal recognition of individual objects. Recent findings in patients with simultanagnosia indicate improved global perception at smaller spatial distances between local elements of hierarchical organized complex visual arrays. Global perception thus does not appear to be an all-or-nothing phenomenon but can be modified by the spatial relationship between local elements. The present study aimed to define characteristics of a general principle that accounts for improved global perception of hierarchically organized complex visual arrays in patients with simultanagnosia with respect to the spatial properties of local elements. In detail, we investigated the role of the number and size of the local elements as well as their relationship with each other for the global perception. The findings indicate that global perception increases independently of the size of the global object and depends on the spatial relationship between the local elements and the global object. The results further argue against the possibility of a restriction in the attended or perceived area in simultanagnosia, in the sense that the integration of local elements into a global scene is impaired if a certain spatial “field of view” is exceeded. A possible explanation for these observations might be a shift from global to local saliency in simultanagnosia.
Journal Article
Visual perception without awareness in a patient with posterior cortical atrophy: Impaired explicit but not implicit processing of global information
A patient with progressive posterior cortical atrophy (PCA)
was examined on several tests of visual cognition. The patient
displayed multiple visual cognitive deficits, which included
problems identifying degraded stimuli, attending to two or more
stimuli simultaneously, recognizing faces, tracing simple visual
stimuli, matching simple shapes, and copying objects. The patient
was also impaired in identifying visual targets contained at
the global level within global–local stimuli (i.e., smaller
letters that compose a larger letter). Although the patient
denied any conscious awareness of the global form, he nevertheless
displayed a normal pattern of global interference when asked
to identify local level targets. Thus, the patient processed
the global information despite not being consciously aware of
such information. These results suggest that global–local
processing can take place in the absence of awareness. Possible
neurocognitive mechanisms explaining this dissociation are
discussed. (JINS, 2002, 8, 461–472.)
Journal Article
Some workmen can blame their tools: artistic change in an individual with Alzheimer's disease
The rapidity and extent of change in artistic ability is indicative of a process above and beyond normal ageing, particularly given his relatively young age at onset.14 Over an interval of 5 years, there has been an objective deterioration in the quality of artwork produced. There is evidence of decline in the ability to represent spatial relations between features and objects, and proportion and perspective. Backgrounds are either simpler or no longer provided. Brushwork has also become more coarse and the artist now solely produces pencil line drawings. Studies of spontaneous drawing in Alzheimer's disease have highlighted the presence of perceptual and executive visuospatial deficits.6 Neuropsychological measures of visuoperceptual and visuospatial function show significant decline, as a component of the global cognitive impairment. At the time of diagnosis there was evidence of difficulty achieving a fully structured percept, but only mildly impaired perception of complex spatial relations. However, assessment 4 years later revealed some impairment of visual sensory capacities, severe difficulty with perceptually degraded stimuli, impaired point localisation, and very poor judgment of relative object position. This deterioration in the ability to process perceptual and spatial information is of primary importance in accounting for the observed artistic change. It should also be noted that visual recognition memory was well preserved in both assessments. Whilst memory for words may often decline in advance of memory for faces,15,16 relatively well-preserved visual memory function may have been a factor in [William Utermohlen]'s continued ability to paint.
Journal Article
Ettlinger revisited: the relation between agnosia and sensory impairment
The concept of agnosia as a higher order functional impairment, which can occur in the absence of low level visual perceptual deficits, continues to provoke debate. This controversy is complicated by the fact that, on close examination, agnosic patients do tend to have some perceptual difficulties. Thus the issue centres around the question as to whether these deficits play a causal part in the aetiology of agnosia or whether they are functionally independent, with both impairments resulting from the substantial cerebral lesions involved in agnosia. In 1956, Ettlinger published a study in which he compared the performance of patients with visual recognition deficits and patients with posterior brain lesions whose recognition abilities were intact. He argued that visual perceptual problems could not explain the recognition deficit in agnosia as he saw far worse perceptual impairments in patients who did not experience any problems in visual recognition. Although the logic of Ettlinger's argument is not disputed, some criticisms have arisen concerning the study, such as the fact that his experimental group did not include a truly object agnosic patient. In addition, Ettlinger's visual-sensory assessment can no longer be considered comprehensive in the light of present day knowledge of the cerebral visual apparatus. This study therefore investigated three (prosop)agnosic patients and five patients with unilateral brain lesions without recognition deficits on an extensive battery of visual sensory tests. The results support Ettlinger's original claim that (in some cases) agnosia cannot be explained as resulting from lower level visual impairments.
Journal Article
Case Studies in the Neuropsychology of Vision
Many unique and now classic cases are described in the nine chapters of this book. Deficits of early stages of vision such as those involved in the processing of motion and colour are described in the first two chapters by Charles Heywood, Josef Zihl, and Alan Cowey. Deficits of later stages of vision such as those involved in object recognition that result in visual agnosia are described by Glyn Humphreys, and Jules Davidoff and Elizabeth Warrington. Within this group of cognitive deficits are the fascinating category-specific agnosias in which recognition of one category of objects, usually those that are living, is selectively impaired while the recognition of artefactual objects is preserved. This topic is covered by Emer Forde, who has an upcoming book on the topic that is co-authored with Humphreys. Optic aphasia is another intriguing type of agnosia whereby an object cannot be recognized through the visual modality but motor actions that are appropriate to the object can be produced. Jane Riddoch discusses this problem in her chapter. Marlene Behrmann and her colleagues take on the issue of whether there are shared processes and representations in visual perception and imagery. Edward DeHaan and his colleagues describe covert recognition in prosopagnosia, another category-specific agnosia in which faces cannot be visually recognized. Finally, Martha Farah provides her broad theoretical perspective to the mix in the last chapter. Farah is one of the few researchers in this field who has taken on the task of summarizing many case studies and providing a theoretical framework for their interpretation.
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